Polyesters, polyamides, polyolefins and other representative thermoplastic resins (Throughout this specification, the simple term "resin" will sometimes be used to refer to such "thermoplastic resins".) have excellent physical and chemical properties and are therefore widely used as fibers, films and other molded products. Despite their superior properties, however, such resins are associated with such undesirable problems as poor workability during molding, or poor manageability as a result of unsatisfactory sliding properties of the molded products themselves during their handling.
Several techniques have already been developed in order to solve these problems. For example, numerous methods have been proposed for improving the surface slidability of molded products by including fine particles in the resins to provide suitable irregularities on the surfaces of the molded properties, and a few of these methods are being employed. Taking polyesters as an example, there is a process whereby silicon oxide, titanium dioxide, calcium carbonate, talc, kaolinite or other inactive inorganic particles are added to the polyesters (see, for example, Japanese Unexamined Patent Publication No. 55-133431), and a process whereby heat-resistant polymer particles such as silicon particles or polystyrene particles are added to the polyesters (see, for example, Japanese Unexamined Patent Publication No. 3-115354).
The aforementioned thermoplastic resins are also used, in a wide range of industrial fields, as modified resins endowed with new properties such as flame retardance, electrostatic properties, dyeability, dyeing clarity and heat resistance which cannot be obtained by resins alone, while still maintaining the original excellent properties of the resins. Techniques for producing resins which meet the demands for such a wide range of uses include, in addition to the inclusion of particles as mentioned above, also methods of copolymerization or blending of the resins with different functional substances for different purposes, and such methods provide good results in terms of high performance and high functionability of the final products.
One process which has been attempted as a technique for giving various functions to thermoplastic resins involves providing a mixing apparatus in the polymer transport line of the molding step for reeling or film formation, to uniformly add and mix different additives with the resins. However in most cases, since the thermoplastic resins are highly viscous when in a molten state, addition and mixture of particulate, liquid or pasty additives directly with the resins results in poor dispersability of the additives in the resins and insufficient quality when used as fibers or films.
Thus, in order to improve the dispersability of additives, inclusion of such additives to resins has been accomplished by a method wherein a "master batch" containing a given additive at high concentration is prepared first and kneaded into the molten resin to improve the dispersability of the additive in the resin. According to this method, preparation of the master batch allows the viscosity and surface tension of the master batch to be adjusted to match that of the resin with which it will be kneaded, for kneading of the master batch with the resin, and thus allows the state of admixture to be improved. In the kneading systems for such master batch processes, static mixing apparatuses used as part of the transport line up to molding of the resin are a publicly known type of mixing apparatus. An example of a known process where such a static mixing apparatus is employed is one in which two types of chips, for the resin and the master batch, are blended prior to the kneading extruder for melting of the chipped resin, and after loading and melting, they are passed through a static mixing apparatus and sent to a reeling machine (see Japanese Unexamined Patent Publication No. 59-126457). According to this process, however, mixture of the resin is accomplished not dynamically but statically, and therefore since there is no external energy during mixing there has been a limit to the extent of admixture of the additives. As a result, the density and quality of obtained products have been non-uniform, the dispersion of additives in resins has been inadequate, and their uses have been limited to a narrow range including those which do not demand high performance products.
Incidentally, systems for polymerization of thermoplastic resins are gradually shifting from the conventional batch systems to continuous polymerization systems. This is because continuous polymerization systems give products with less quality variation than batch polymerization systems, are suitable for mass production of specific grades over long periods, and are overwhelmingly advantageous in terms of cost. In addition, products discharged from batch polymerization systems have lower intrinsic viscosity with time, more quality variation between different batches resulting in poor color, and more variation in stocked materials and quality variation between batches due to differences in reaction conditions, etc. In order to solve these problems, such as the problem that once the resin has been chipped it must be blended with chips obtained from a different batch, continuous polymerization systems achieve low quality variation by keeping constant and consistent control of the operating conditions in each step. Also, when disturbances or other variations occur, it is relatively easy with continuous polymerization systems to minimize changes in resulting products with time during the polymerization step by appropriately controlling the process conditions so as to eliminate such disturbances. In addition, while it is difficult to increase the performance of existing equipment for each batch in a batch system, in the case of continuous polymerization systems the advantages have been multiplied by recent progress in technological innovations which allow scaled-up production.
Despite the advantages described above, continuous polymerization systems have a disadvantage in that they are not adaptable for small-scale production of different product types. In particular, for production of modified thermoplastic resins containing various modifiers such as those mentioned above, changing the type of modifier requires cleaning of the entire massive continuous polymerization apparatus, resulting in a huge loss which includes that of polymer waste, cleaning chemicals and time. With the rapid progress in scaled-up size and product diversity in recent years, these disadvantages of continuous polymerization systems have become ever more serious.
In light of this background, the greatest technical issue in the field of producing resin compositions has recently become that of determining how to achieve production with increased dispersion of modifiers in different modified thermoplastic resin compositions without losing the cost merit of continuous polymerization, and how to diversify for different final needs.
In addition, with the development of continuous polymerization systems it has recently become practical to accomplish direct film formation and spinning for formation of films and spinning of fibers. With developing techniques, continuous polymerization-based direct film formation and direct spinning systems are being employed in the attempt to eliminate steps which are essential in batch systems, such as transport of the fully polymerized polymer to the film formation or spinning step after first being chipped, stored in a silo and dried, and with the purpose of further rationalizing of the processes.
Nevertheless, loading of different additives just prior to the direct film formation line or direct spinning line for the purpose of achieving different grades is associated with a serious drawback in that the appearance of disturbances is directly produced in the products when the density and quality are non-uniform. Because of this drawback, the step of direct film formation or direct spinning from continuous polymerization currently involves a serious problem whereby it is impossible to eliminate streaks which often occur with time in the polymer quality during direct transport of the fully polymerized polymer through the withdrawal line to the molding step.
In order to solve this problem, it has become common to employ kneading systems which melt the master batch with the molten polymer in the polymer withdrawal line of the continuous polymerization system. In such systems, the use of static mixing apparatuses for admixture of master batches and polymers for production of modified polyesters of superior quality has become a publicly known technique, as has been proposed in Japanese Unexamined Patent Publication No. 59-126457 and Japanese Examined Patent Publication No. 4-14128.
Nevertheless, as was already mentioned, mixing techniques using such static mixing apparatuses involve no application of external energy during the mixing and thus have a major disadvantage in that they cannot be used for strong mixing, as opposed to techniques where the mixing is accomplished with forced external power. In addition to such problems, static mixing apparatuses also have another drawback in that, although the polymer is mixed by dividing the polymer flow in the plane perpendicular to the polymer flow, thus allowing a degree of uniform dispersion of the additive in that plane, no technique yet exists for elimination mixing streaks which occur in the direction of polymer flow. In other words, it is currently the case that there is absolutely no effect for elimination of streaks which occur with time in the direction of polymer flow.
Reexamination of master batch systems from this standpoint highlights the problem with master batch systems, that it is impossible to avoid streaks which occur with time in the polymer due to density and quality variations in the modifier-containing thermoplastic resin, i.e. the master batch, and rotation cycle streaks generated by the rotation cycle of the pump used for transport of the base polymer and modifier-containing polymer. Thus, master batch systems which employ static mixing apparatuses have not been suitable for direct film formation and direct spinning from continuous polymerization where changes occurring with time appear as variations in the quality of the products, and therefore a technique has been desired which would resolve this issue.
As has already been mentioned, continuous polymerization systems have the disadvantage of being unsuitable for small-scale production, but at present there is still an increasing need for higher functioning and diversification of resins with modifiers in continuous polymerization systems. Because of this situation, techniques such as proposed in Japanese Examined Patent Publication No. 46-37767 have been developed as attempts at production for diverse grades. According to such techniques, multiple molten polymers at different polymerization stages in a continuous polymerization apparatus made from a multistage polymerization can are appropriately taken out from the polymerization can and blended, and used in composite form or alone as single polymers to obtain polyesters with different polymerization degrees, or the polyesters are combined in an appropriate fashion. Multi-grade techniques have also been proposed which include the procedure of this technique for production of composite fibers with latent crimping performance both efficiently and in combinations of ample variety.
It is true that this technique is advantageous in allowing compound fibers with combinations of different polymerization degrees to be manufactured by adjustment of the polymerization degree or mixing ratio of the polymer upon branching of the polymer composing the composite fibers from multiple polymerization cans. However, this technique merely combines a plurality of polymers at different polymerization stages in the manufacturing process for a single polymer produced in a continuous manner by continuous polymerization and is therefore limited in terms of diversification of grade, while various modifiers cannot be added for higher grade diversification and differentiation of polymer functions.
In an attempt to improve these drawbacks of the prior art processes, the present inventors have endeavored to provide a modified thermoplastic resin composition sufficiently flexible for diversification and multigrade production, as well as a production process therefor, by means of a mixing system which can give highly dispersable thermoplastic resins exhibiting no streaking with time and which allows uniformly dispersed mixing of various modifiers therein for adaptability to direct film formation and direct spinning.
In other words, the present invention provides a modified thermoplastic resin composition with excellent dispersability of modifiers in the thermoplastic resin and with good mold working properties or functions for molding of resin products in addition to high dispersability without producing streaks with time, as well as a process for its preparation. As a result, particularly in cases where the obtained thermoplastic resin composition is to be supplied from the continuous polymerization step to a direct film formation or direct spinning step, it is possible to accomplish continuous production of modified thermoplastic resins which are free from such changes which occur with time.